'Ontic' here loosely refers to interpretations that treat the quantum state as a real physical entity. This is the same as 'realism', but that term is ambiguous so it isn't necessarily a bad thing that there is this redundancy. The opposite of an ontic interpretation is a non-realist interpretation--one that takes the quantum state to not be a real entity itself. A non-realist interpretation is a position about explanation. It holds that there is a different explanation of events that is better than the realist explanation, and that these explanations cannot both be true.
'Epistemic' here refers to interpretations that take the quantum state to be 'states of belief about the system', which is horribly confusing. I don't like this definition. Belief doesn't actually factor into physics like this, so something is not right here.
Let's consider another term instead: probabilistic. The term 'epistemic' pretty much refers to an explanation that is probabilistic. With phase space probability distribution in classical thermodynamics, there is some collection of outcomes which are not known deterministically in advance, but we can write up a function which will hold true of the distribution of outcomes we see. Regardless of whether or not each individual outcome is able to be found deterministically, we can use a probability distribution to understand the whole collection of outcomes. This is a probabilistic physical prediction, and Spekkens is simply arguing that a quantum state is just the same kind of thing as this phase space distribution.
The tricky thing about all of this is that there's an assumption that these probabilistic (epistemic) theories are non-real theories, but that simply isn't the case. What makes these probability distrutions true are "chance set-up" situations. There has to be something that sets up the distribution to be predictable by a probability distribution. If there is no chance set-up, there's no reason to believe that a probabilistic prediction will be true. Nancy Cartwright writes much better than I about this aspect of probabilistic laws (1999). These chance set-ups, then are the description of the world that physics brings. Probability distributions are not lacking in ontological committments, since they require that the world be set up in this certain way. Maybe this is a less satisfying ontological claim than one about the existence or non-existence of something, but it undoubtedly is an ontological claim that physics makes.
So probabilistic (epistemic) theories are not necessarily non-real about quantum states. They are only non-real if they provide an alternate explanation for quantum effects compared to the the interpretation of quantum mechanics which takes quantum states to be real entities. Spekkens argues that there is alternate explanation that is better, we just don't know it yet. But he has no reason to argue that this is actually true. Remember, the 'Toy Theory' does not actually replace quantum mechanics in explanatory power. At best, it gives an account for what the new explanation would look like, but this is not enough to claim that interpretations that take quantum states to be real are incorrect.
The tricky thing about all of this is that there's an assumption that these probabilistic (epistemic) theories are non-real theories, but that simply isn't the case. What makes these probability distrutions true are "chance set-up" situations. There has to be something that sets up the distribution to be predictable by a probability distribution. If there is no chance set-up, there's no reason to believe that a probabilistic prediction will be true. Nancy Cartwright writes much better than I about this aspect of probabilistic laws (1999). These chance set-ups, then are the description of the world that physics brings. Probability distributions are not lacking in ontological committments, since they require that the world be set up in this certain way. Maybe this is a less satisfying ontological claim than one about the existence or non-existence of something, but it undoubtedly is an ontological claim that physics makes.
So probabilistic (epistemic) theories are not necessarily non-real about quantum states. They are only non-real if they provide an alternate explanation for quantum effects compared to the the interpretation of quantum mechanics which takes quantum states to be real entities. Spekkens argues that there is alternate explanation that is better, we just don't know it yet. But he has no reason to argue that this is actually true. Remember, the 'Toy Theory' does not actually replace quantum mechanics in explanatory power. At best, it gives an account for what the new explanation would look like, but this is not enough to claim that interpretations that take quantum states to be real are incorrect.
In fact, Spekkens seems to overlook the possibility that quantum mechanical theorizing is in some ways epistemic and that the wave function is also a real physically existing thing. This is the view I like best. In a sense, it's not quantum mechanics that has failed to understand some phenomena, but really it is classical mechanics that has failed. Classical mechanics asks where a particle is, and only gets a probability distribution, which quantum mechanics gives. What is obviously in partial belief (and probabilistic) is our knowledge of particle positions, so this is in a sense uncontrovertably epistemic. But, at the same time, what provides the probability distributions accurately? Quantum mechanics does.
Spekkens asks us to look for a third theory which can explain why our knowledge of particle positions seem to always be in partial belief, and therefore probabilistic, but the answer is something more obvious than what he proposes. In fact, quantum mechanics is a theory which correctly describes the real world, and provides correct probability distributions for our lack of knowledge of the positions of particles.
Update: Upon reflection, I realize that this treatment of the dichotomy is not entirely fair to it. I think that the point I made here is still valid, I just don't think the case is closed on this dichotomy. It's absolutely a false dichotomy; there is just more to say about it that is less dissmissive. I'll write up another post soon...
Update: Upon reflection, I realize that this treatment of the dichotomy is not entirely fair to it. I think that the point I made here is still valid, I just don't think the case is closed on this dichotomy. It's absolutely a false dichotomy; there is just more to say about it that is less dissmissive. I'll write up another post soon...
